What is the INR formula?

The INR equals the patient PT ratio raised to the power of the reagent ISI: INR = (patient PT / mean normal PT)^ISI. The mean normal prothrombin time (MNPT) is the geometric mean of PTs from at least 20 healthy donors for that reagent and instrument combination.

Why is the ISI needed?

Different thromboplastin reagents have different sensitivities to the clotting factor reductions caused by warfarin. The ISI calibrates each reagent against the international reference thromboplastin, so applying it makes results comparable between laboratories regardless of which reagent was used.

Can I enter a PT ratio directly?

Yes. If you already have the PT ratio, set the mean normal PT to 1 and enter the ratio as the patient PT, since the ratio of those equals the ratio itself. Otherwise enter both prothrombin times and the tool computes the ratio for you.

What is a typical therapeutic INR?

Most warfarin indications target an INR of 2.0 to 3.0, with some mechanical heart valves targeting 2.5 to 3.5. The tool flags values within and above the common 2.0 to 3.0 range as a convenience, but the correct target depends on the indication.

Does the INR apply to direct oral anticoagulants?

No. The INR is validated only for monitoring vitamin K antagonists such as warfarin. It is unreliable for direct oral anticoagulants like apixaban or rivaroxaban, which require drug-specific assays where monitoring is needed.

INR from PT Ratio & ISI Calculator

The INR standardises prothrombin time results so that warfarin monitoring is comparable between laboratories that use different reagents. This tool computes the INR from the patient and mean normal prothrombin times and the reagent’s International Sensitivity Index.

How it works

The calculation has two steps:

PT ratio = patient PT (s) / mean normal PT (s)
INR      = PT ratio ^ ISI

The mean normal prothrombin time (MNPT) is the geometric mean of prothrombin times from at least 20 healthy donors for the specific reagent and analyser. The ISI, printed on the reagent insert, calibrates that thromboplastin against the international reference preparation, so raising the ratio to the ISI removes the between-reagent variation.

Example and notes

A patient prothrombin time of 24 seconds with a mean normal prothrombin time of 12 seconds gives a PT ratio of 2.0. With a sensitive modern reagent of ISI 1.0, the INR is 2.0; with a less sensitive reagent of ISI 1.2, the same ratio gives an INR of about 2.30. This is exactly why the ISI matters — the raw ratio is identical but the standardised result differs. Always use the ISI for the exact reagent lot in use, and remember the INR is valid only for vitamin K antagonists, not direct oral anticoagulants.